The present invention relates to an alignment apparatus used in semiconductor manufacturing apparatuses, various machine tools, and the like.
Conventionally, the following two examples are known as an alignment apparatus used in semiconductor manufacturing apparatuses, various machine tools, and the like. The first example is an alignment apparatus which drives a movable member by converting rotation of a motor into translation using a feed screw. The second example is an alignment apparatus which uses a linear motor in a driving system, and also uses a non-contact guide that supports a movable member using the pressure of compressed fluid, as described in Japanese Patent Laid-Open No. 59-178507.
However, in the first example, the reproducibility of alignment precision in the feed and reverse directions is poor due to the influences of friction or the like of the feed screw. More specifically, the alignment precision suffers, owing to nonlinear factors such as friction. In order to improve the alignment precision, a nonlinear compensation element such as for friction compensation or the like must be added to a control system. In such a case, since the arrangement of the overall system is complicated, it becomes hard to adjust a controller. Also, when the characteristics of the nonlinear element have changed due to aging of the frictional force as a result of the use of the apparatus, the control system and mechanical components must be re-adjusted.
The second example is directed to an alignment apparatus based on drive control using a linear motor. This apparatus suffers a lower support rigidity in the feed direction than that using the feed screw or the like. FIG. 8 shows the arrangement of a general position control loop of the conventional alignment apparatus. A detector 805 detects the state variables (XO, YO) of a movable member (X-Y stage) 804 in response to command variables (XI, YI), and feeds them back to a controller 802. In general, since the driving system using a linear motor normally has an insufficient rigidity in the feed direction, if the natural frequency of the alignment mechanism falls within the control frequency band of alignment, it may impair the alignment precision. Hence, when a linear motor is used in the alignment mechanism, it is important to increase its rigidity. When the control frequency of the linear motor agrees with the natural frequency of the alignment apparatus in the feed direction, they cause resonant vibrations, and make the alignment control unstable.
In this alignment apparatus, the linear motor serving as the driving source is independent from any position detection mechanism of the motor shaft corresponding to a rotary encoder, resolver, or the like, and a position detector is attached to the movable member to be controlled and detects its movement to feed it back to the controller. In this case, the influences of local resonant vibrations of an attachment member of the position detector may be superposed on the vibration characteristics of the overall system.
To compensate for the low support rigidity of the movable member in the feed direction and to avoid the influences of resonance in the control frequency band, it becomes difficult to appropriately set setting parameters (gains) of the control loop. As a consequence, the settling time is prolonged due to the influences of residual errors with respect to the target position, and vibrations produced due to the movement of the movable member, resulting in a long alignment time.